UV-induced and photo-sensitized changes of hyaluronic acid and collagen, UV-absorbing species in hyaluronic acid, and interactions between collagen and hyaluronic acid have been investigated. UV (254 nm)-induced degradation of hyaluronic acid from the bovine vitreous was found to be highly sensitive to ionic strength. Rate of degradation was inversely proportional to ionic strength. The UV absorption spectra of UV (240-300 nm)-absorbing species, which were not removed by dialyzing against buffers with high ionic strength, were found pH-dependent. The possibility that the UV absorbing-amino acid tryptophan may act as a sensitizer for UV degradation of hyaluronic acid has been studied. The tryptophan-containing peptide Lys-Trp-Lys did not show photo-sensitized degradation upon photo-excitation at 295 nm. However, the photooxidation product of tryptophan, N-formyl-kynurenine, exhibited a high photo-sensitizing action with 365 nm UV irradiation. When acid-soluble type I collagen from rat tail tendon was mixed with hyaluronic acid in 0.05% acetic acid solution, precipitation occurred. The precipitate was not completely soluble in 0.1M acetate buffer at pH 4, but soluble in 0.4M buffer. UV (275 nm)-irradiation of tyrosine residues in collagen in the absence of oxygen induced an increase in viscosity, complete inhibition of native-type fibril formation and enhancement of two fluorescences at 410 and 430 nm. UV (365 nm)-irradiation of Benzo(a)pyrene bound to collagen caused partial photo-fragmentation of collagen.